1. Field of the Invention
The invention relates to an ultrasonic linear array transducer for utilization in catheter instruments and to methods of manufacturing curved linear array transducers for catheter-mounted applications so as to comply with acoustic and surgical specifications relating to such products.
2. Related Art
Intraluminal catheter-mounted ultrasonic transducers of several different kinds and forms have been widely discussed in the literature. Usually, the transducing device is attached at, or in the vicinity of, the tip end of the catheter, and wave radiation is then directed outwardly from the catheter either in a transverse radial mode, longitudinal radial mode, forwardly directed mode, circular radial mode or a combination thereof. The catheter instrument is basically comprised of an elongated tube having, at one end thereof, an insertion tip wherein the transducer is mounted and terminated at the other end by a handle wherein are gathered the bending controls for the instrument. Catheters designed for use in cardiac examinations commonly have a diameter ranging from 3 to 10 French catheter scale, meaning that the external diameter of the inserting tube does not exceed a value of, respectively, 1 to 3.3 mm. The tube is approximately of 1 to 2 meters long and is connected to the imaging transducer of the system by a flexible coupler which is capable of causing bending of the transducer tip in two or four directions.
An example of an imaging catheter is disclosed by U.S. Pat. No. 3,938,502 to Bom, wherein the catheter is a circular radially emitting device. The elements of the transducer array are disposed around the catheter circumference to form a circular scanning image. Such product is particularly well suited to arterial applications and more specifically, to examining for arteriosclerosis in plaque wherein a sectional view of the artery is required.
In U.S. Pat. No. 4,605,009 to Pourcelot, there is disclosed an endoscope probe for imaging an internal wall of human body. In order to widen the angle of view of the instrument, a curved linear array transducer is associated with optical visualization means. Assembly of the transducer includes the provision of a wire interconnection between the transducer piezoelectric member and a rigid printed circuit board (PCB) located at the bottom side of the transducer, and soldering of coaxial cables to the PCB to complete the interconnection. Such a construction is voluminous and prohibitive of any miniaturization of transducer for implementation thereof in intraluminal probes.
Another endoscopic ultrasonic instrument of interest is disclosed in U.S. Pat. No. 5,291,893 to Slayton wherein a linear array transducer is provided at the distal end of an endoscope or catheter. The transducer array is mounted in a tip housing that is capable of rotating around its longitudinal axis thereby providing rotation of the scanning plane without moving the catheter. However, the transducer array construction method includes the provision of an interconnection PCB located beneath the transducer assembly where coaxial cables and piezoelectric contact wires are soldered. Making the interconnection in this way wastes the considerable space behind the array so miniaturization of transducer tip diameter is difficult and the result unreliable. Furthermore, the rotation of the transducer tip with regard to the rest of the catheter requires a dynamic seal at the junction between the two moving parts. This junction is a collection site for dust accumulation and biological contamination so the device can not be reused in surgical operations. Further, with regard to the manufacturing cost of the device, there is no corresponding disposable diagnostic instrument available on the market.
PCT Patent WO92/03972 to Crowley relates to an imaging system combined with an insonifier for detecting the position and orientation of the associated catheter device. The insonifier utilizes ultrasound with a frequency that can be detected by the catheter sensor. The energy transferred between the insonifier and the catheter is maximized when the two devices face each other, so that by identifying the orientation of one of the devices, the position of either can be determined. Because the frequency of transmit energy from the insonifier should be at least in the bandwidth of catheter device, the separation distance between the two devices is limited. The external insonifier, when used as described in the patent, is difficult to control and in actual practice, the alignment of the insonifier relative to the catheter remains approximate.
In PCT Patent WO98/33430 to Curley, there is disclosed a catheter-mounted transducer array for ultrasonic imaging. The transducer includes an end portion surrounding the array and providing a speed of sound therein greater than or equal to 1250 msxe2x88x921. The transducer azimuth axis is parallel to the longitudinal axis of the catheter and is molded or sheathed to have an elongated cylindrical tip portion having the same proportions as those of the catheter. The Curley patent discloses two alternative methods of making surrounding material for the array transducer, viz., a molding method and a pre-formed thermoplastic material. The surrounding material is characterized by its cylindrical cross-section without regard to the elevation focus of the array. Accordingly, the speed of sound in the surrounding material must be close to that of the propagation medium so as not to modify the focus of transducer. Otherwise, no details as to the placement of the transducer in molding method are provided. The pre-formed sheathing method provides for the assembly of the transducer array into the pre-formed sheath using a glass tube associated with a mandrel and subsequent heating to eliminate air gaps between the transducer array and the polymer sheath. One of the important drawbacks of this method concerns the difficulty of the insertion of the transducer array into the polymer sheath without causing damage to the array due to pressure exerted thereon. Further, the heat applied to the polymer sheath to shrink the sheath onto the array will inherently transfer heat to the transducer array and this heat can depolarize the piezoelectric structure thereof. Another aspect of the Curley patent that is not clearly described concerns the angular orientation of the array in the case where the polymer sheath is provided with an acoustic window which is designed with a particular radius (concave or convex). It is evident, that with this arrangement, the position of the elevation plane of the array with regard to the geometry of lens is of critical importance.
In accordance with the invention, there is provided a novel method for making ultrasonic imaging transducer arrays for catheter use which enables implementation of curved or linear array shapes as well as provides for detection of the array orientation by a radio-opacity method described below. The method of the invention is also compatible with low cost fabrication requirements and thus finds application in the high volume disposable catheter market.
According to one aspect of the invention, a linear ultrasonic array transducer assembly is provided for intraluminal catheter use wherein a curved linear ultrasonic transducer array is mounted at or on the distal end of an elongated catheter so as to provide expanded viewing of an organ to be imaged.
Another aspect of the invention concerns the provision of an ultrasonic transducer array construction which provides optimized acoustic performance as well as compact sizing so as to be compatible with conventional vascular insertion techniques. The associated transducer comprises a sandwich of a piezoelectric member, backing block or member, matching layer, focusing lens and bonded interconnection means.
A further aspect of the invention concerns the flexibility of the transducer array. The flexibility provided enables the transducer array to bend slightly in order to facilitate insertion thereon when embodied as a curved linear array. The transducer array then recovers its initial shape once arriving at the desired site.
Yet another aspect of the invention concerns the integration of stiffening dorsal elements or xe2x80x9csticksxe2x80x9d that provide elasticity to the transducer array during an insertion operation as well as other advantages.
Still another aspect of the invention concerns the provision of dorsal elements or xe2x80x9csticksxe2x80x9d which are opaque to radio frequencies and can be viewed during the imaging process so as to enable the examiner to detect the orientation of the transducer array during placement and use thereof.
An additional aspect of the invention concerns the provision of a polymer sheath or covering over the transducer array. In one preferred method of this aspect or embodiment, the molding of the polymer sheath is omitted and the transducer array is inserted into the tube of the catheter. The tube terminates in a flexible portion at which the array is located. Expansion of a flexible array receiving portion of the tube is provided in order to facilitate insertion of the transducer array into this flexible tube portion without excessive constraining forces being exerted thereon.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.